1918] Packard: Molluscan Fauna from San Francisco Bay 211 
entitled, ‘‘A report upon the physical conditions in San Francisco 
Bay, based upon the operations of the United States Fisheries’ 
Steamer ‘Albatross’ during the years 1912 and 1913,’’ by F. B. Sum- 
ner, G. D. Louderback, W. L. Schmitt, and E. C. Johnston (1914). 
San Francisco Bay is ‘‘an irregular body of water surrounded by 
marshes and prolonged into estuaries . . . extending from the mouth 
of Sonoma Creek at the extreme northern end to the mouth of the 
so-called ‘Coyote River’ at the extreme southern.’’ As thus defined 
it includes San Pablo Bay. It has a length of 52 statute miles and a 
maximum width of 11.5 miles and an estimated area of 400 square 
miles. 
The Sacramento and San Joaquin rivers empty into the upper 
part of San Francisco Bay through Carquinez Strait. There are 
several small streams, most of which are intermittent, that contribute 
considerable volumes of water to the bay during the rainy season. 
The total discharge of these streams affects the temperature and 
salinity of the waters of the bay, besides bringing in sediments that 
are in part deposited within San Francisco Bay. 
The depth of San Francisco Bay ranges up to 63 fathoms encoun- 
tered in the narrower portion of Golden Gate. It has been shown by 
Sumner et al. (p. 21) that only 18.4 per cent of the total area of the 
bay has a depth greater than 5 fathoms. The mean depth has been 
computed to be 22.7 feet. The deeper waters are found in the middle 
division of the bay within the Golden Gate and in Raccoon Strait 
just north of Angel Island. The deeper waters of the upper and 
lower divisions of the bay are confined to narrow central channels. 
It may be well to define here the divisions of the bay since these 
terms will appear frequently throughout the paper. Sumner et al. 
(1914, p. 22) recognize three divisions of San Francisco Bay. The 
‘“upper’’ one includes San Pablo Bay; the ‘‘middle’’ one extends 
from a line passing through the points of San Pedro and San Pablo 
to a line drawn from the Ferry Building to the Goat Island Light; 
the third or ‘‘lower’’ division lies to the south of this latter line. 
The mean tidal range for the entire bay throughout the month is 
given as 4.52 feet. The actual extremes during the course of the year 
are much greater, ranging from 0.4 to 7.8 feet at Fort Point within 
the Golden Gate. 
The rate of the tidal currents was determined for a number of 
localities to be about 1.4 knots per hour at a distance of a few feet 
below the surface. It was estimated that the mean rate of water flow 
